There are interactive displays which comprise e.g. a projector or a Liquid Crystal Display (LCD) panel to form an image on a surface, a camera or other optical detector behind the surface, and a computer vision circuitry for detecting user input.
There are numerous different technologies for optical multi-touch detection. Many of these are described in the publication “ThinSight: Versatile Multi-touch Sensing for Thin Form-factor Displays” by Steve Hodges, Shahram Izadi, Alex Butler, Alban Rrustemi and Bill Buxton, see http://www.billbuxton.com/UISTthinSight.pdf. Optical touch detection is advantageous for a variety of reasons including ability to detect multiple touch points or areas on the display. However, the optical recognition of touching is exposed to illumination related changes in the environment. Typically, infrared (IR) light is used to illuminate the touch screen and the reflecting or shadowing of the IR light is measured. The operation of such touch screens depends, for example, on the frequency and amplitude distribution of ambient light, direction of ambient light, and reflectance and size of user's fingers.
It is an object of the present invention to avoid problems associated with prior art and/or to provide an alternative to existing technology.